The background is described in connection with an improved modulator useful in cellular telephony applications using a modified Direct Digital Synthesizer (DDS). It should be understood, however, that the principles disclosed may apply to a wide array of applications where component space is limited and a digital implementation of the signal modulation circuitry would help decrease overall component count and/or limit system board space requirements.
Inductors, capacitors and other passive components are essential elements of many electronic devices. Such components are used to perform a variety of functions such as filtering, inter-stage impedance matching, and decoupling of AC and DC signals. For example, in cellular communications such components are used to transmit, receive and filter a modulated analog signal carrier into its audible voice signal counterpart. With the move to smaller and lighter devices, a premium is often placed on the size and number of components in the design.
Modulation is the function which imposes certain characteristics to an electromagnetic signal based on a set of rules and the data to be transmitted. One common modulation technique is known as Frequency Shift Keying (FSK). With FSK modulation, an output signal is switched between two separate frequencies with a higher frequency Nhigh representing a "mark" frequency and a lower frequency Nlow representing as the "space" frequency. Thus, a data train can be reconstructed from an analog signal by modulating the signal in time between "marks" and "spaces."
The basic FSK modulation scheme 10 is illustrated in FIG. 1 wherein the digital data stream 12 (101101) and its corresponding FSK counterpart 14 are shown. Note that the switching from a "mark" to a "space" occurs almost instantaneously causing a series of abrupt discontinuities 16 in the data stream 12. The discontinuities 16 in the data stream 12 consume a frequency spectrum of a theoretical infinite range since the rate of change of the signal 12 over time approaches infinity. This rate of change is often referred to as the slew rate and expressed as the mathematical equivalent dv/dt.